A bridge (or switch) may be used to provide a “bridging” or (“switching”) function between two or more local area networks (LANs) or end stations. Typically, the bridge is a computer and includes a plurality of ports that is coupled via LANs either to other bridges, or to end stations such as routers or host computers. To avoid the formation of loops, most bridges and switches execute a spanning tree protocol which allows them to calculate an active network topology that is loop-free (i.e., a tree) and yet connects every pair of LANs within the network (i.e., the tree is spanning). To obtain the information necessary to run the spanning tree protocol, bridges exchange special messages called configuration bridge protocol data unit (BPDU) messages or simply BPDUs. In addition, as will be understood by those skilled in the art, the network may be configured with one or more virtual local area networks (VLANs), generally sharing one spanning tree.
In certain networks, a multiple spanning tree (MST) protocol (MSTP) may be used to allow for multiple instances of spanning trees for different VLANs (that is, each VLAN is assigned to a particular spanning tree instance). Conventionally, however, the standardized MST protocol (a standard of the Institute of Electrical and Electronics Engineers, or the “IEEE”, namely, 802.1Q-2005) does not allow a bridge to send control traffic (BPDUs) on a per-VLAN basis. Instead, IEEE standard MSTP currently requires control traffic to be sent on a per-physical link/port basis. As such, a single BPDU representing multiple STP instances and thus multiple VLANs is typically sent from each port of a bridge.
A link that interconnects a switch/bridge into a device (e.g., service devices, such as firewalls) that does bridging between VLANs and then back into the same switch is viewed by the MST protocol as a simple looped-back link, particularly due to the BPDUs (and their association with each of the VLANs) being sent out and received back at the same port. Accordingly, the switch disables or “blocks” the looped ports from each other (that is, to prevent looping traffic from the switch to itself), as will be understood by those skilled in the art.